JP3033920B2 - Optical frequency conversion method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical frequency conversion method for converting signal light of a certain optical frequency into light of another frequency.

[0002]

2. Description of the Related Art As an optical frequency conversion method, a method using a second-order or third-order optical nonlinearity and a method using a semiconductor optical amplifier are known. Since the present invention solves the problem of the optical frequency conversion method using the third-order optical nonlinearity of an optical fiber, this method will be described as a conventional technique.

When a plurality of frequency lights are input to an optical fiber, new frequency lights are generated due to third-order optical nonlinearity. For example, the input optical frequency is denoted by _{f 1, f 2, f 3} , nonlinear polarization components occurs in the frequency position of _{f 1 + f 2 -f 3 =} f 4, thereby new frequency light is generated. Here, the optical frequency f ₁ has been modulated, the frequency light f _2, f ₃ is assuming that a non-modulated, the same modulation signal to the frequency f ₁ is the optical frequency f ₄ newly generated is ing. In other words, the optical frequency f ₄ can be regarded as frequency-converted light from the frequency f _1. This situation is f
_The same applies if ₂ is a modulated signal light and f ₁ and f ₃ are non-modulated light, or f ₃ is a modulated signal light and f ₁ and f ₂ are non-modulated light.

Generation power P of nonlinear light of frequency f _4
4 depends on the optical fiber structure, input light power, input light frequency, etc., and is specifically expressed as follows.

P ₄ = (1024π ⁶ / n ⁴ λ ² c ² ) (Dχ) ² (P ₁ P ₂ P ₃ / A ₂ ) e- ^αL ｛(1-e- ^αL ) ² + 4e- ^αL sin ² ( Δk ^L / 2)｝ / (α ² + Δk ² ) (1) In the above equation, n is a refractive index, λ is a light wavelength, c is a light speed, D
Is the degeneracy factor, χ is the third-order nonlinear constant, P ₁ , P ₂ , P
₃ is the input power of light of frequencies f ₁ , f ₂ and f ₃ , respectively, A
Is the mode cross-sectional area, α is the loss factor of the optical fiber, and L is the optical fiber length. Δk is given by Δk = k (f ₁ ) + k
(F ₂ ) −k (f ₃ ) −k (f ₄ ) (however, k (f ₁ ), k (f ₂ ), k (f ₃ ), k (f ₄ )
Are the propagation constants of light at frequencies f ₁ , f ₂ , f ₃ , and f ₄ respectively.
And represents the amount of phase mismatch between the generated nonlinear polarization and the propagating nonlinear light. Equation (1) is derived from the following paper.

[0006] KO Hill, DC Johnson, BS Kawas
aki, and RI MacDonald, "CW three-wave mixing in
single-mode optical fibers ", Journal of Applied P
hysics, vol. 49, pp. 5098-5106 (1978). (1) According In the formula, as .DELTA.k small, generation efficiency of the frequency light f ₄ is large, it becomes maximum when the .DELTA.k = 0. this is,
This is because when Δk = 0, nonlinear lights generated at various points in the optical fiber are always added in the same phase. Therefore,
In order to realize highly efficient frequency conversion, it is necessary to satisfy Δk = 0. The condition of Δk = 0 is called a phase matching condition.

[0007]

However, since chromatic dispersion generally exists, the condition of Δk = 0 cannot be satisfied as it is. This situation is concretely expressed as follows.

First, each propagation constant is developed around an optical frequency f ₀ as follows.

K (f) = k (f ₀ ) + (f−f ₀ ) (dk / df) (f ₀ ) +1/2 (ff− ₀ ) ² (d ² k / df ² ) (f ₀ ) +1/6 (f−f ₀ ) ³ (d ³ k / df ³ ) (f ₀ ) When this is further rewritten, k (ω) = k (ω ₀ ) + (ω−ω ₀ ) (1 / Vg ) − (Ω−ω ₀ ) ² (λ ² / 4πc) Dc (ω ₀ ) +1/6 (ω−ω ₀ ) ³ (λ ⁴ / 4π ² c ² ) ｛(2 / λ) D _c (ω ₀ ) + DD _c / dλ)｝. Here, Vg is the group velocity at f ₀ , D _c is the chromatic dispersion value of the optical fiber, and the optical frequency f is rewritten at each frequency ω. When Δk is written using this expression, it is as follows.

Δk = {(2πλ ² ) / c｝ P _c (f ₀ ) (f ₁ −f ₃ ) (f ₂ −f ₃ ) + (λ ⁴ π / 3c ² )} (2 / λ) P _c (F ₀ ) + (dD _c / dλ)｝ ｛(f ₁ −f ₀ ) ³ + (f ₂ −f ₀ ) ³ − (f ₃ −f ₀ ) ³ − (f ₄ −f ₀ ) ³ . 2) As can be seen from this equation, Δk = 0 is not obtained because the chromatic dispersion D _c exists. However, when the frequency difference of the input light is small, Δk ≒ 0, and the phase matching condition is approximately satisfied. Therefore, when the input frequency difference is small, highly efficient frequency conversion is possible. For example, if a fiber having a length of 10 km is used, the frequency-converted light is efficiently generated in a range of about several tens of GHz, and the generation rate is deteriorated in a wider range. When the input frequency difference is small, the frequency difference between the generated nonlinear light and the input light is also small. That is, in the optical frequency conversion using the nonlinearity of the optical fiber, there is a problem that the frequency difference that can be converted is limited to, for example, several tens of GHz.

The present invention has been made in view of such circumstances, and an object thereof is to provide an optical frequency conversion method having a wide conversion range.

[0012]

To achieve the above object, according to an aspect of, the present invention, to the optical fiber, by inputting the signal light and the pumping light of a predetermined frequency of the frequency f _s, frequency <br/> number the optical frequency conversion method of generating light of f _c, the upper
As the pump light, the original signal light frequency f _s ,
Wavenumber f _c, with respect to the zero dispersion frequency f ₀ of the optical fiber, frequency
A first pump light having f _p1 = 2f ₀ -f _s, the frequency f _p2 =
And to use a second pump light of 2f ₀ -f _c.

[0013]

[0014]

According to the present invention, the original signal light frequency f _s, strange
換先frequency f _c, versus zero dispersion frequency f ₀ of the optical fiber
And, a first pump light frequency f _p1 = 2f ₀ -f _s, frequency
The number f _p2 = 2f ₀ second pump light and an optical fiber -f _c
Is entered. As a result, the phase matching condition is always satisfied, and a frequency conversion with a wide conversion range can be performed .
Optical frequency conversion to any frequency with one optical fiber
It becomes possible .

[0015]

[0016]

Embodiment 1 FIG. 1 is a diagram showing a configuration example of an optical frequency conversion device to which an optical frequency conversion method according to the present invention is applied.
1 is a pump light source, 2 is an optical coupler, and 3 is an optical fiber for frequency conversion.

[0017] In this configuration, the optical fiber 3 through the optical coupler 2 the pump light of the signal light and the frequency f _p of the frequency f _s
By entering into, and to generate light of a frequency (2f _p -f _s) by utilizing the third-order nonlinearity of the optical fiber 3. At this time, the frequency of the pump light is arranged so as to match the zero dispersion wavelength of the optical fiber 3.

FIG. 2 shows the relationship between the zero-dispersion wavelength of the optical fiber and the input optical frequency. Hereinafter, the first embodiment of the method of the present invention will be described in more detail with reference to FIG. Here, the frequency f _s of the signal light f _3, the frequency f _p of the pump light and f _1.

According to the description in the conventional example, the pump light is also light having frequencies f ₁ and f ₂ . For this two-frequency optical input, f ₄ = f ₁ + f ₁ −f ₃ = 2f ₁
New frequency light is generated in the frequency position of the -f _3.

Here, consider Δk under the input condition of FIG. In equation (2), f ₀ is an arbitrary frequency.
Now, let this be the zero dispersion frequency of the optical fiber. Then, since D _c (f ₀ ) = 0, Δk = {(λ ⁴ π) / 3c ² } (dD _c / dλ)} (f ₁ −f ₀ ) ³ + (f ₂ −f ₀ ) ³ − (f ₃₋ f ₀ ) ³ − (f ₄ −f ₀ ) ³ ｝ (3) Further, since the pumping light which also serves as a frequency light of f ₁ and f ₂ are located in the zero dispersion frequency, f ₁ = f ₂ =
f ₀ . When this is substituted, Δk = {(λ ⁴ π) / 3c ² } (dD _c / dλ)} −
(F ₃ −f ₀ ) ³ − (f ₄ −f ₀ ) ³ }. Furthermore, since it is _{f 4 = 2f 1 -f 3 =} 2f 0 -f 3, the _{f 4 -f 0 = f 0 -f} 3. When this is substituted, Δk = 0. That is, it is understood that the phase arrangement condition is satisfied in the frequency arrangement of FIG.

The above relationship does not depend on the frequency position of f ₃ . Regardless of the frequency of f ₃ , the phase matching condition is always satisfied if the pump light is located at the zero dispersion frequency. Therefore, the frequency conversion of FIG. 2 enables frequency conversion with a wide conversion range. Note that the conversion destination frequency at this time is 2f ₀ −f ₃ , and the conversion frequency difference is 2 (f
₃ = f ₀ ).

[0022]

Embodiment 2 FIG. 3 is a view for explaining a second embodiment of the method of the present invention, and shows the relationship between the zero dispersion wavelength of the optical frequency conversion optical fiber used and the input optical frequency. This embodiment is different from the above-described first embodiment in that although the first embodiment can perform frequency conversion with a wide conversion range, the conversion destination frequency is fixed, while one optical fiber can be used for arbitrary conversion. The point is that it can be converted to optical frequency.

That is, the conversion destination frequency is fixed at a frequency position symmetrical to the original signal light with respect to the zero dispersion frequency of the optical fiber. Therefore, when it is desired to change the conversion destination frequency, it is necessary to separately prepare an optical fiber having a zero dispersion wavelength corresponding thereto. However, in consideration of practicality, it is desirable that one optical fiber can convert to an arbitrary optical frequency. Therefore, in the present embodiment, the configuration is such that frequency conversion to an arbitrary optical frequency is possible.

[0024] Specifically, with respect to optical frequency conversion optical fiber, by inputting the pumping light of the signal light and the predetermined frequency of the frequency f _s, Upon generating light of frequency f _c, as the pump light, also With respect to the signal light frequency f _s , the conversion destination frequency f _c , and the zero dispersion frequency f ₀ of the optical fiber,
A first pumping light of a frequency f _p1 = 2f ₀ -f _s, are configured to use a second pump light frequency f _p2 = 2f ₀ -f _c.

Hereinafter, a second embodiment of the method of the present invention will be described in more detail with reference to FIG. In FIG. 3, the original signal light frequency for converting f _1, a f ₄ and the destination frequency.

Unlike the first embodiment, f ₄ is not necessarily at a symmetrical position with respect to f ₁ and the zero dispersion wavelength, and may be any frequency. In some cases, both f ₁ and f ₄ may be on the higher frequency side or lower frequency side than the zero dispersion wavelength. Here, in order to perform frequency conversion from f ₁ to f _4, to enter two pump light satisfy the following frequency relationship. f ₂ −f ₀ = − (f ₁ −f ₀ ) That is, f ₂ = 2f ₀ −f ₁ f ₃ −f ₀ = − (f ₄ −f ₀ ) That is, f ₃ = 2f ₀ −f ₄ where f ₂ Is the optical frequency of the first pump light, f ₃ is the optical frequency of the second pump light, and f ₀ is the zero dispersion frequency of the fiber. In other words, the first pump light is disposed at a frequency position symmetrical to the original signal light with the zero dispersion wavelength interposed therebetween.

By substituting the above frequency relation into equation (3), the result Δk = 0 is obtained. That is, it can be seen that the frequency arrangement of FIG. 3 satisfies the phase matching condition. This relationship is always satisfied by using pump light having a frequency corresponding to any frequency of f ₁ and f ₄ . Therefore, according to the present embodiment, the optical frequency conversion to an arbitrary frequency is possible.

[0028]

As described above, according to the present invention ,
Optical frequency conversion of a wide conversion range can be realized , and
By using only one optical fiber, the optical frequency to an arbitrary frequency
Wave number conversion becomes possible . Therefore, there is an advantage that it can be applied to a frequency conversion element in a system using optical frequency multiplexing, particularly in the field of optical communication and optical information processing.

[0029]

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of an optical frequency conversion device to which an optical frequency conversion method according to the present invention is applied.

FIG. 2 is a frequency allocation diagram of the first embodiment.

FIG. 3 is a frequency allocation diagram of a second embodiment.

[Explanation of symbols]

1. Pump light source, 2. Optical coupler, 3. Optical fiber for frequency conversion.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-31489 (JP, A) JP-A-3-226131 (JP, A) Electronics Letters, Vol. 24 No. 24 pp. 1528-1529 (24th November 1988) IEEE Journal of Quantum Electronics, Vol. QE-23 No. 7 pp. 1205-1210 (Jully 1987) Optics Letters, Vol. 7 No. 10 pp. 489-490 (October 1982) Optics Letters, Vol. 13 No. 12 pp. 1117-1119 (December 1988) Journal of Lightwave Technology, Vol. 9 No. 3 pp. 356-361 (March 1991) Journal of Lightwave Technology, Vol. 9 No. 10 pp. 1330-1334 (October 1991) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/35-1/365 JICST file (JOIS)

Claims (1)

(57) [Claims] To 1. A fiber, by inputting the signal light and the pumping light of a predetermined frequency of the frequency f _s, the frequency
The optical frequency conversion method of generating light of f _c, as the pump light, the original signal light frequency f _s, the destination
With respect to the frequency f _c and the zero dispersion frequency f ₀ of the optical fiber ,
A first pumping light of a wave number f _p1 = 2f ₀ -f _s, the frequency f _p2
= Optical frequency conversion method, which comprises using a second pump light of 2f ₀ -f _c.